Photoconductive switch package

ABSTRACT

A photoconductive switch is formed of a substrate that has a central portion of SiC or other photoconductive material and an outer portion of cvd-diamond or other suitable material surrounding the central portion. Conducting electrodes are formed on opposed sides of the substrate, with the electrodes extending beyond the central portion and the edges of the electrodes lying over the outer portion. Thus any high electric fields produced at the edges of the electrodes lie outside of and do not affect the central portion, which is the active switching element. Light is transmitted through the outer portion to the central portion to actuate the switch.

RELATED APPLICATIONS

This is a continuation of U.S. patent application Ser. No. 12/775,156,filed May 6, 2010 incorporated herein by reference, which claimspriority to U.S. Provisional Application Ser. No. 61/176,375, filed May7, 2009, titled “Photoconductive Switch Package” incorporated byreference.

GOVERNMENT RIGHTS

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the U.S. Department of Energy andLawrence Livermore National Security, LLC, for the operation of LawrenceLivermore National Laboratory.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to photoconductive switches andmore particularly to optically initiated silicon carbide (SiC) and otherhigh voltage switches.

2. Description of Related Art

Particle accelerators, for example dielectric wall accelerators (DWAs),critically depend on high voltage, high current, fast closing switchesthat can be activated with nanosecond precision. Photoconductiveswitches offer the most potential in terms of handling such highvoltages and high currents with minimum inductance, rapid closure,precise temporal control and the possibility of long life.Photoconductive switching is a technique in which optical energy isapplied to a semiconductor material, and the optical energy createscarriers within the semiconductor, rendering it conductive.

The materials that have been used to date for photoconductive switchapplications include Silicon and Gallium Arsenide (GaAs). The difficultywith these materials is that various failure mechanisms occur, even atmodest parameters. Further, the optical absorption depth for thesematerials is low, so the carriers are forced to flow in a very thin areaof the material bulk just below the surface. The principal issue withphotoconductive switching has been short lifetime resulting fromoverstressing current and voltage conditions.

Additionally, silicon carbide (SiC) has long been a promisingalternative candidate for use as a photoconductive switch material. Onlyvery recently, however, has this particular material been commerciallyavailable in sizes and purity that merit application as a high voltageswitch. SiC material has high dielectric breakdown strength, greaterthan that of most solid materials (about 4MV/cm); high thermalconductivity (comparable to that of copper); and low optical absorption.Thus, with the availability of single crystalline Silicon Carbide, a newclass of switches is possible.

While promising, even SiC is subject to failure due to high electricfields locally produced where the electrodes separate from contact withthe photoconductive substrate. A prior art photoconductive switch,having a SiC photoconductive substrate and two opposing electrodes,typically has a meniscus formed at the metal contact between theelectrode and substrate surfaces. The meniscus refers to a small blob ofindium solder that is used to bond an electrode to the substratesurface. The magnitude of the electric field on the contact surfaces hasa spike in magnitude at the triple points. The triple point is a regionwhere the electrode edge, the SiC wafer, and another material like aninsulating oil all come together. The indium solder has some verticalheight to it that permits oil to come in contact with both the electrodeand wafer, causing, the triple point. Various methods have been employedto reduce and minimize these fields at such “triple points,” includingfilling the space between the electrode and substrate with a highpermittivity material. However, there still an electric field spike,albeit with less magnitude, at the triple point of electrode-substrateseparation.

Copending U.S. patent application Ser. No. 11/586,468 describes aphotoconductive switch with a photoconductive substrate having opposingelectrode-contacting surfaces and a facet optically connectable to anoptical source for receiving optical energy; two electrodes electricallyconnected to the electrode-contacting surfaces of the substrate, forapplying a potential across the substrate; and two field-grading linersformed on the substrate surrounding the electrode-contacting surfaces,for grading the electric fields therealong. The field-grading liners maybe adjacent to the electrode perimeters, or they may be adjacent to thesubstrate perimeter, but they are integrally formed into the substrate.The field-grading liners may be made of high permittivity materials orconductive or semi-conductive materials; a suitable material is siliconnitride. The liners may be formed as doped sub-surface layers of thesubstrate, extending into the substrate about 1 micron deep. Optionally,the substrate can be a multilayer having at least two photoconductivelayers separated by a divider layer, with the divider layer composed ofconductive and semi-conductive materials. While the field-grading linersmay reduce the electric field effects, they add a level of complexitysince they must be fabricated into the photoconductive substrate.Another problem with resistive liners is that they usually must becustom configured for a particular pulse width of applied voltage andwill not work for arbitrary waveforms.

The photoconductive substrate shown in Ser. No. 11/586,488 may have twoopposing concavities and the two electrodes may have convex surfacescontactedly seated in the two concavities These electrode shapes couldbe used in the present invention.

What is needed therefore is a photoconductive switch for high voltageapplications such as for particle accelerators, preferably implementedwith SiC or other photoconductive materials such as GaN, that minimizesor at least reduces the high magnitude electric fields at the points ofelectrode-substrate separation. A switch design that does not requirealteration of the photoconductive substrate would be highlyadvantageous.

BRIEF SUMMARY OF THE INVENTION

The invention is a photoconductive switch and method of making same. Theswitch is formed of a substrate that has a central portion of SiC orother photoconductive material and an outer portion of cvd-diamond orother suitable material surrounding the central portion. Conductingelectrodes are formed on opposed sides of the substrate. The dimensionsof the electrodes are greater than the dimensions of the central portionof the substrate so that the electrodes extend beyond the centralportion and the edges of the electrodes lie over the outer portion. Thusany high electric fields produced at the edges of the electrodes lieoutside of and are substantially isolated from the central portion,which is the active switching element. Light is transmitted through theouter portion to the central portion to actuate the switch.

The invention includes a photoconductive switch, having a substrate madeup of a photoconductive central portion and a non-photoconductive outerportion surrounding the central portion; and conducting electrodesformed on opposed sides of the substrate, with the electrodes extendingbeyond the central portion and the edges of the electrodes lying overthe outer portion.

The invention also includes a photoconductive switch, having a substratemade up of a photoconductive central portion and an outer portionsurrounding the central portion; the substrate having opposedelectrode-contacting surfaces and a light receiving facet orthogonal tothe electrode-contacting surfaces; conducting electrodes electricallyconnected to the opposed electrode-contacting surfaces of the substratefor applying a potential across the substrate; the dimensions of theelectrodes being greater than the dimensions of the central portion sothat the electrodes extend beyond the central portion and the edges ofthe electrodes lie over the outer portion; wherein the effects of highelectric fields produced at the edges of the electrodes lie outside ofand do not substantially affect the central portion.

The invention further includes a method of making a photoconductiveswitch, by providing a photoconductive central portion; bonding an outerportion thereto to form a switch substrate; and forming electrodes onthe switch substrate, the electrodes extending beyond the centralportion so that the electrode edges he over the outer portion.

Further aspects of the invention will be brought out in the followingportions of the specification, wherein the detailed description is forthe purpose of fully disclosing preferred embodiments of the inventionwithout placing limitations thereon.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1A is a cross-sectional side view of a prior art basicphotoconductive switch.

FIG. 1B is a graph of the operation of a photoconductive switch.

FIGS. 2A, B are cross-sectional views of an upper half of first andsecond exemplary embodiments of a prior art photoconductive switch withfiled-grading liners.

FIGS. 3A, B are a top plan view and a cross-sectional side view of aphotoconductive switch of the invention.

FIG. 4 is an assembly drawing of the photoconductive switch substrate ofthe invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposesthe present invention is embodied in the apparatus and method generallyshown in FIGS. 3A, B and 4. It will be appreciated that the apparatusmay vary as to configuration and as to details of the parts, and themethod may vary as to its particular implementation and as to specificsteps and sequence, without departing from the basic concepts asdisclosed herein.

The photoconductive switch of the present invention is based on thebasic prior art photoconductive switch construction and operation, withenhancements enabling the switch to handle high voltages and highcurrents with minimum inductance, rapid closure, precise temporalcontrol and the possibility of long life. As such, the photoconductiveswitch of the present invention shares much the same construction andfeatures as a basic photoconductive switch, generally having a substrateof photoconductive material between two electrodes. Without opticalenergy injection, i.e. in the dark, the photo-conductive material is aninsulator with a large resistance value (much larger than the circuitimpedance) and thus the switch essentially blocks current flow. Whenoptical energy is injected and absorbed in the photoconductive material,the switch resistance drops to a small value (much less than the circuitimpedance) and the switch conducts current. Thus the basicphotoconductive switch is essentially an optically controlledresistance. The availability of relatively small pulsed lasers or otheroptical sources enable the injection of optical energy in a short pulseso that the switching time between large blocking resistance and smallconduction resistance can easily be several nanoseconds.

FIG. 1A shows the basic features of a prior art photoconductive switch20 formed of a photoconductive substrate 22 with a pair of opposedelectrode-contacting surfaces 24, 26 and a facet 28 orthogonal to thesurfaces 24, 26. Facet 28 optically connectable to an optical, sourcefor receiving optical energy therefrom to actuate the switch. Forexample, a laser or other light source 18 may be coupled to a lightreceiving facet 28 by optical fibers 38. Substrate 22 is typically athin wafer made of SiC or other suitable material. Electrodes 30, 32 areelectrically connected, respectively, to the electrode-contactingsurfaces 24, 26 of the substrate 22, for applying a potential across thesubstrate, as represented by the voltage “V” and ground. When a voltageis applied across electrodes 30, 32, and the switch 20 is “open” or“off,” high electric fields are produced at the edges 34, 36 ofelectrodes 30, 32. Since electrodes 30, 32 are smaller in area thansubstrate 22, edges 34, 36 are on the substrate 22 and the high electricfields may affect the substrate 22, and, e.g., cause breakdown orpremature closing of the switch.

FIG. 1B illustrates the basic operation of switch 20. When no light isinjected into facet 28, substrate 22 is nonconductive so no currentflows and the voltage is held across the switch. When an optical pulsefrom source 18 is injected into substrate 22 through facet 28, switch 20is “closed” or “on,” substrate 22 becomes conducting, switch currentflows, and the voltage across the switch drops. When the light pulseends, switch 20 turns off again. Thus the light source 18 is used tocontrol switch operation. However, the electric fields produced at edges34, 36 of electrodes 30, 32 may cause the switch 20 to close at unwantedtimes because they may lead to breakdown through substrate 22.

FIGS. 2A, B illustrate a pair of prior art switches 40, 42 formed of asubstrate 44 with an electrode 46 on a surface thereof. Field-gradingliners 48 are also formed in the substrate, either at electrode edge orthe substrate edge, to mitigate the electric field effects.

FIGS. 3A, B show a photoconductive switch 50 of the invention. Switch 50is formed of a substrate 52 that has a central portion 54 of SiC orother photoconductive material and an outer portion 56 of cvd-diamond orother suitable material surrounding the central portion 54. Conductingelectrodes 60 are formed on opposed sides (electrode-contactingsurfaces) of substrate 52. The dimensions of electrodes 60 are greaterthan the dimensions of central portion 54 of substrate 52 so thatelectrodes 60 extend beyond central portion 54 and the edges 62 ofelectrodes 60 lie over outer portion 56. Thus any high electric fieldsproduced, at the edges 62 of electrodes 60 lie outside of and do notaffect the central portion 54, which is the active switching element ofswitch 50. In operation, laser or other light is input to switch 50through one of more light receiving facets 58. For example, light from asource 18 may be coupled through optical fibers 38 as in FIG. 1A. Thelight passes through outer portion 56 to photoconductive central portion54 where it actuates the switch as described previously. One or morenon-light receiving facets 66 may be coated with a thin dielectric layer64 to improve optical properties, e.g. produce total internalreflection, i.e. to keep the light inside the switch bottled up so thatit will keep interacting with the switch rather than escaping from theswitch. In FIG. 3A light is introduced into two opposed light receivingfacets 58 so there are two non-light receiving facets 66 while in FIG.3B light is introduced into one light receiving facet 58 so there arethree non-light receiving facets 66 (only one of which is shown).

The material for outer portion 56 of substrate 52 should be a materialwith a higher bulk breakdown strength than the material in the centralportion. It should also be optically transparent to the light used toactuate the switch. In addition it should be thermally conductive toremove heat from the central portion of the switch and it should form astrong electric field tolerant bond to the central portion. Optimally,the material is not photoconductive so it is not the active part of theswitch; it allows the photoconductive active part of the switch tofunction while handling the electric field stresses at the electrodeedges and guides light to the active part.

Cvd-diamond is an excellent material for the outer portion of thesubstrate. The bulk breakdown strength of good quality diamond is about10 MV/cm, compared to 4 MV/cm for SiC, so it can handle the enhancedfield stresses at the electrode edges. Diamond has very large thermalconductivity so it can remove heat from the switches into a region oflow electric field for ultimate removal. Cvd-diamond can be madeoptically transparent so it can act as a light guide for switchillumination to reach the central portion of the switch. Cvd-diamond canform a strong, electric field tolerant bond to SiC since both materialscontain carbon; this joint will resist electrical breakdown.

FIG. 4 illustrates the assembly of a photoconductive switch substrate 70of the invention, substrate 70 is similar to substrate 50, The centralportion 72 of photoconductive material, typically square in shape, isprepared. A pair of outer portion elements 74, of the same width ascentral portion 72 are prepared and bonded to two opposed sides ofcentral portion 72. Then a pair of outer portion elements 76, of a widthequal to that of central portion 72 plus the two elements 74, areprepared and bonded to the remaining two opposed sides of centralportion 72 and the sides of elements 74. When assembled together,elements 74, 76 form the outer portion similar to outer portion 56. Theelectrodes are then applied to the substrate 70. Alternately, all foursides could be grown simultaneously on the central portion so thatindividual pieces are not required.

The invention includes a method of making a photoconductive switch byproviding a photoconductive central portion and bonding an outer portionthereto to form a switch substrate. Electrodes are then formed on theswitch substrate, the electrodes extending beyond the central portion sothat the electrode edges lie over the outer portion.

Preferably, the photoconductive switch of the present invention usesCompensated, Semi-Insulating Silicon Carbide (CSI-SiC) as thephotoconductive substrate, since it is considered the best material forapplication in high power photoconductive switch applications. This isdue to the following reasons. CSI-SiC's very large dielectric strength(3 MV/m) permits very thin layers to support large voltages (e.g. GaAscan only support about 250 kV/cm). CSI-SiC switches thus require reducedlevels of optical closure energy since the required optical closureenergy scales as the thickness of the CSI-SiC material. CSI-SiC's largedark resistance (10 Ohm-cm) permits low rates of voltage application orcharging (maximum GaAs resistivity is about 10⁹ Ohm-cm). CSI-SiC's largethermal conductivity permits high average power operation withoutthermally induced conduction (GaAs thermal conductivity is only 10% ofSiC). The compensated nature of CSI-SiC, i.e. the dopant concentration,enables the design of recombination times, optical absorption depths,and thus current densities. GaN is also a preferred material for theswitch.

The substrate may be a compensated, semi-insulating material selectedfrom SiC (e.g. 4 h SiC or 6 h SiC), GaN, AlN, and diamond, andpreferably having a hexagonal crystal structure and cut in a planeselected from the A-Plane, C-Plane and M-plane. The semi-insulating SiCor other material is preferably doped with at least one of the followingdopants: Boron, Vanadium, Nitrogen, Aluminum, Phosphorus, Oxygen,Tungsten and Zinc.

The photoconductive switches preferably use wide band gap material withbelow band gap illumination. The photoconductive substrate may be agreater-than-2.5 eV wide band gap material.

The invention thus provides a photoconductive switch to handle highvoltages and high currents with minimum inductance, rapid closure,precise temporal control and the possibility of long life. Thephotoconductive switches have a configuration with the contacts orelectrodes placed in relation to the photoconductive material so thatthe effects of high electric field stress at the electrode edges arereduced or eliminated. The switches have numerous applications,including high energy particle accelerators, including dielectric wallaccelerators (DWAs). The switch could be used in a compact protonaccelerator for medical therapy.

Although the description above contains many details, these should notbe construed as limiting the scope of the invention but as merelyproviding illustrations of some of the presently preferred embodimentsof this invention. Therefore, it will be appreciated that the scope ofthe present invention fully encompasses other embodiments which maybecome obvious to those skilled in the art, and that the scope of thepresent invention is accordingly to be limited by nothing other than theappended claims, in which reference to an element in the singular is notintended to mean “one and only one” unless explicitly so stated, butrather “one or more,” All structural and functional equivalents to theelements of the above-described embodiment that are known to those ofordinary skill in the art are expressly incorporated herein by referenceand are intended to be encompassed by the present claims. Moreover, itis not necessary for a device to address each and every problem soughtto be solved by the present invention, for it to be encompassed by thepresent claims. Furthermore, no element or component in the presentdisclosure is intended to be dedicated to the public regardless ofwhether the element or component is explicitly recited in the claims. Noclaim element herein is to be construed under the provisions of 35U.S.C. 112, sixth paragraph, unless the element is expressly recitedusing the phrase “means for.”

I claim:
 1. A method of making a photoconductive switch, comprising:providing a substrate comprising: a photoconductive central portion; anda non-photoconductive outer portion surrounding the central portion; andforming conducting electrodes on opposed sides of the substrate, withthe electrodes covering the central portion and extending beyond thecentral portion, wherein each electrode comprises an edge, wherein eachof the edges of the electrodes lie over the outer portion.
 2. The methodof claim 1, further comprising forming the outer portion of a firstmaterial with a higher bulk breakdown strength than a second material inthe central portion.
 3. The method of claim 2, wherein the firstmaterial is optically transparent to light used to actuate the switch.4. The method of claim 3, wherein the outer portion is formed of amaterial that is sufficiently thermally conductive to effectively removeheat from the central portion of the switch.
 5. method of claim 4,wherein the outer portion is formed of a material that forms a strongelectric field tolerant bond to the central portion.
 6. The method ofclaim 1, wherein the central portion is formed of a wide band gapmaterial.
 7. The method of claim 6, wherein the wide band gap materialis a greater-than-2.5 eV wide band gap material.
 8. The method of claim1, wherein the central portion is formed of a compensated,semi-insulating material.
 9. The method of claim 8, wherein the materialis selected from SiC, GaN, AlN, and diamond.
 10. The method of claim 9,wherein the material is SiC doped with at least one of the followingdopants: Boron, Vanadium, Nitrogen, Aluminum, Phosphorus, Oxygen,Tungsten and Zinc.
 11. The method of claim 1, wherein the outer portionis formed of cvd-diamond.
 12. The method of claim 11, wherein thecentral portion is formed of SiC.
 13. The method of claim 1, furthercomprising electrically connecting a voltage source to the electrodes.14. method of claim 1, further comprising operatively positioning alight receiving facet such that it is orthogonal to the opposed sides onwhich the electrodes are formed for receiving optical energy to actuatethe switch.
 15. The method of claim 14, further comprising providing alaser or other light source configured to be optically coupled to thelight receiving facet.
 16. A method of making a photoconductive switch,comprising: providing a substrate comprising: a photoconductive centralportion comprising a first material; and an outer portion surroundingthe central portion, wherein the substrate has a firstelectrode-contacting surface on a first side of the substrate and asecond electrode-contacting surface on a second side of the substratedirectly opposite of the first side, wherein the substrate furtherincludes a light receiving facet orthogonal to the firstelectrode-contacting surface and the second electrode-contactingsurface; and electrically connecting a first conducting electrode to thefirst electrode-contacting surface and a second conducting electrode tothe second electrode-contacting surface, wherein the first conductingelectrode and the second conducting electrode are configured forapplying a potential across the substrate, wherein the dimensions of theelectrodes are greater than the dimensions of the central portion sothat the electrodes cover the central portion and extend beyond thecentral portion and all of the edges of the electrodes lie over theouter portion, wherein the effects of high electric fields produced atthe edges of the electrodes lie outside of and do not substantiallyaffect the central portion, wherein the outer portion is formed of asecond material that has a higher bulk breakdown strength than the firstmaterial in the central portion and that is optically transparent tolight used to actuate the switch, and the central portion is formed of awide band gap material.
 17. The method of claim 16, wherein the outerportion is made of cvd-diamond and the central portion is made of SiC orGaN.
 18. The method of claim 16, further comprising providing a voltagesource electrically connected to the electrodes and a laser or otherlight source optically coupled to the facet.